Integrated circuit having electrically isolatable test circuitry

ABSTRACT

Special test circuitry in an IC for wafer level testing selectively connects the specialized test circuitry to the functional circuitry during wafer test. Following wafer test the special test circuitry is electrically isolated from the functional circuitry and power supplies such that it does not load functional circuit signals nor consume power.

This application is a divisional of prior application Ser. No.10/345,648, filed Jan. 16, 2003, currently pending, which claimspriority under 35 USC § 119(e)(1) of provisional application No.60/349,590, filed Jan. 18, 2002.

BACKGROUND OF THE INVENTION

Today ICs are designed to include test circuitry, such as scan and BuiltIn Self Test (BIST), that can be used to test the IC at all levels ofassembly and manufacturing, i.e. wafer test, packaged IC test, systemintegration test, and field test. In order to reuse the test circuitryin such a manner, the test circuitry must be designed as an integral andactive part of the IC. Being an integral part of the IC, the testcircuitry is connected to the functional circuitry to be tested and alsoconnected to the IC power supply rails.

While this is the way traditional test circuitry is designed into ICs,there are some types of specialized test circuitry included in ICs thatonly participate in wafer level testing. This specialized test circuitryadvantageously allows wafer level testing to be performed using lowercost testers and with higher precision, especially the testing ofsensitive analog circuits. Like other scan and BIST test circuitry, thisspecialized test circuitry is conventionally designed to be connected tothe functional circuitry it will test and to the IC's power supplies.However, unlike the scan and BIST circuitry, the specialized testcircuitry is only usable at the wafer test level since the die padsrequired for accessing the specialized test circuitry are typically notbonded out to package pins.

U.S. Pat. No. 5,578,935 teaches a method and apparatus of testing acircuit under test by embedding an integrated strobed comparator testcircuit in the IC and connecting an input of the comparator to theoutput of a circuit under test in the IC. The integrated strobedcomparator and circuit under test are also connected to an externaltester for power, reference voltage inputs, and test input stimulus andoutput response signaling. The test arrangement of FIG. 1 of U.S. Pat.No. 5,578,935 allows the tester, the circuit under test and comparatorwithin the IC to interact together according to a described successiveapproximation algorithm of FIG. 2 to achieve the test. The motivationand advantages for embedding the comparator into the IC are that theembedded comparator minimizes the effect of stray capacitance andinductance on a signal under test.

BRIEF SUMMARY OF THE INVENTION

The present invention describes a method and apparatus using specialtest circuitry in an IC for wafer level testing but without having topermanently connect the specialized test circuitry to the functionalcircuitry after wafer test is complete. The advantage brought forth bythe present invention is that following wafer test the special testcircuitry is electrically isolated from the functional circuitry andpower supplies such that it does not load functional circuit signals norconsume power.

The integrated circuit of the present invention provides functionalcircuitry and test circuitry on the same substrate. The functionalcircuitry has first input and output signal leads connected to firstinput and output signal bond pads and first power supply terminalsconnected to first power bond pads. The functional circuitry is adaptedto produce a test response signal at a first output signal bond pad fortesting of the functional circuitry in response to a test stimulussignal being applied to a first input signal bond pad.

The test circuitry has second input inputs and output signal leadsconnected to second input and output signal bond or test pads and secondpower supply leads connected to second power bond or test pads. Thesecond leads and bond or test pads are separate from the first leads andbond pads. The first leads and bond pads and the second leads and bondor test pads are adapted to be selectively connected together during atest to operate both the functional circuitry and the test circuitry totest the operation of the functional circuitry with the test circuitry.A One second input test signal bond pad is adapted to receive the testresponse signal from the first output signal bond pad, another secondinput reference signal bond pad is adapted to receive a test comparisonsignal, and a the second output bond pad signal lead provides a testpass/fail response signal in response to the test comparison signal andthe test response signal being received at the second input outputsignal bond pads pad.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of a test arrangement connected to a knownintegrated circuit.

FIG. 2 is a block diagram of an integrated circuit constructed accordingto the present invention.

FIG. 3 is a block diagram of a test arrangement according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

For the purpose of simplifying the description of the present invention,the use of a type of the above-mentioned special circuitry, as describedin U.S. Pat. No. 5,578,935, will be used. While this one type of specialtest circuitry will be used to describe the advantages of the invention,it should be understood that this is only for exemplary purposes anddoes not limit the scope of the invention and its applicability to otherspecial test circuitry types.

Test arrangement 100 of FIG. 1 of the present invention illustrates asimplified version of the test arrangement of FIG. 1 of U.S. Pat. No.5,578,935. In present FIG. 1, IC 102 relates to circuit 226 of U.S. Pat.No. 5,578,935, FIG. 1, and includes the circuit under test (CUT) 104(202 in U.S. Pat. No. 5,578,935) and integrated strobed comparator 106(206 in U.S. Pat. No. 5,578,935). In present FIG. 1, external tester 140relates to external tester circuit blocks 200, 208, 220, and 222 of U.S.Pat. No. 5,578,935, FIG. 1. With this relationship established, allfurther reference to FIG. 1 in this description will be to FIG. 1 of thepresent specification, unless clearly stated otherwise.

IC 102 of FIG. 1 is assumed to be a die being tested either on wafer orafter singulation. IC 102 has a V+ power supply pad 114, a V− powersupply pad 116, a test response output pad 120, a comparator strobeinput pad 126, a comparator voltage reference input pad 124, afunctional output pad 122, and a test stimulus input pad 118. All thepads, except for the functional output pad 122 in this example, areconnected to the tester 140 to allow the tester to power up and testcircuit 102. As can be seen, the CUT 104 and comparator 106 of circuit102 are both powered by the V+ and V− power supply pads 114,116, viainternal power bussing rails 112 and 110 respectively.

The output 108 from CUT 104 is connected to a first input of thecomparator 106, to the functional output pad 122, and perhaps to othercircuits within IC 102. The second input of comparator 106 is connectedto the voltage reference output 136 from tester 140. The strobe input ofcomparator 106 is connected to a strobe output 138 from the tester 140.In response to the strobe input 138 from tester 140, the comparatoroutputs response to the tester's response input 132. The CUT 104receives stimulus input from the tester's stimulus output 134.

During test, the tester 140 inputs a repetitive stimulus input 134 toCUT 104 to cause the CUT 104 to output 108 a periodic waveform tocomparator 106. Comparator 106, when strobed, outputs a digitizedresponse to tester's response input 132. In one aspect of the test, asdescribed further in U.S. Pat. No. 5,578,935, the test proceeds based ona successive approximation algorithm whereby the tester increases thevoltage reference level 136 to comparator 106 if the strobed responseinput 132 is a logic zero and decreases the voltage reference level 136to comparator 106 if the strobed response input 132 is a logic one. TheIC 102 passes or fails the test based on the digitized response input132 received by the tester 140.

In IC 102, the comparator 106 is permanently connected to the powersupplies 114 and 116 pads that are also connected to the CUT 104. Thisis important. Whenever the CUT 104 is energized, the comparator 106 isalso energized. Comparator 106 therefore consumes power duringfunctional operation of CUT 104 and may, due to a defect in thecomparator circuit, actually render IC 102 non-functional or functionalbut at a reduced or degraded level.

The comparator 106 is permanently connected to the output of the CUT104. This is also important. Comparator 106 therefore provides someamount of loading to the output of CUT 104, which may increase powerconsumption of circuit 102 and/or effect the quality of the CUT output108 at functional pad 122. As will be described in detail below, thepresent invention provides solutions to the above mentioned power andloading concerns when special circuitry (comparator 106) is connected tofunctional circuitry (CUT 104) and power supply rails (112 and 110) ofan IC 102 during its test.

In FIG. 2, an IC 202 incorporates the improvements of the presentinvention. IC 202 is identical to IC 102 with the following exceptions.(1) The permanent connection shown in FIG. 1 between the CUT 104 output108 and the first input of comparator 106 has been removed, thusisolating the CUT output from the comparator 106 first input load. (2)The first input to comparator 106 has been connected to a separate andadditional test (T) pad 204 on IC 202. (3) The V+ and V− power supplyconnections of FIG. 1 between comparator 106 and the V+ and V− pads114,116 have been removed to where only the CUT 104 is connected to andpowered by the V+ and V− power pads 114,116. (4) The V+ and V− powersupplies for comparator 106 have been connected to separate andadditional V+ and V− test power supply pads 206,208.

In FIG. 3, a modified version of the test arrangement 300 depicts IC 202configured for testing. Test arrangement 300 is identical to the testarrangement 100 with the following exceptions. (1) An externalconnection 302 has been formed between the existing V− pad 116 and theadded V− pad 208 to provide the low-level supply voltage to comparator106 from tester 140. (2) An external connection 304 has been formedbetween the existing V+ pad 114 and the added V+ pad 206 to provide thehigh-level supply voltage to comparator 106 from tester 140.

An external connection comprising connection 308, signal conditioner310, and connection 306 has been formed between the functional outputpad 122 and the added test (T) pad 204. The signal conditioner 310 is anactive or passive circuit that can be used, if necessary, for matchingthe output impedance of functional output pad 122 to the input impedanceof test input pad 204. If it is not necessary to use signal conditioner310, then a direct connection may be formed between functional outputpad 122 and test input pad 204. The test performed in the testarrangement 300 of FIG. 3 can be the same as described in regard to FIG.1 and further in U.S. Pat. No. 5,578,935, and, in at least one aspect,can be based on a successive approximation algorithm.

The IC 202, when being tested, is provided with external connectionsthat couple comparator 106 to power supplies, tester signaling, and CUT104, but when not being tested the comparator 106 can be completedisolated from power supplies, tester signals, and CUT 104 by simplyremoving the external connections. Thus the present invention providesfor connecting special test circuitry to functional circuitry, powersupplies, and testers during test but advantageously also provides forcompletely isolating special test circuitry from functional circuitry,power supplies, and testers when testing is not being performed.

After IC 202 is tested, such as after the test bond pads 120, 124, 126,204, 206 and 208 are not wire bond connected to leads on the IC leadframe and IC 202 is encapsulated, connections 302, 304, 306, 308, andsignal conditioner 310 can be removed and prevented from contacting IC202, leaving pads 204, 206, 208 free to contact other pads or the leadframe leads. The special test circuitry therefore exists within IC 202to be advantageously used at a targeted test level (i.e. wafer test) butafter being used is rendered separate and isolated and can be madeinaccessible, such as by covering the test bond pads 120, 124, 126, 204,206, and 208 with encapsulation material, so as to avoid the concernsover power and loading as previously stated.

While the special test circuitry has been described herein as being acomparator for use in testing an analog signal output from a circuitunder test, it should be understood that the special test circuitrycould be any type of test circuitry (digital or analog) that issimilarly used to test other circuits under test (digital or analog).Other types of special test circuitry will, when not being tested, notbe connected to power supplies, other circuit pads, to testers, or tocircuits under test.

1. A process of testing an integrated circuit comprising: A. forming asubstrate of semiconductor material; B. forming functional circuitry onthe substrate, the functional circuitry having a stimulus input bondpad, a functional output bond pad, a first positive power supply bondpad, and a first negative power supply bond pad; C. forming testcircuitry on the substrate, the test circuitry having a reference inputtest pad, a test input test pad, a response output test pad, a strobeinput test pad, a second positive power supply test pad separate fromthe first positive power supply bond pad, a second negative power supplytest pad separate from the first negative power supply bond pad, and acomparator having inputs connected to the test and reference input testpads, power leads connected to the second positive and second negativepower supply test pads, a strobe input connected to the strobe inputtest pad, and an output connected to the response output test pad; andD. connecting the functional output bond pad to the test input test padand the first power supply bond pads to the second power supply testpads external of the substrate only during testing of the functionalcircuitry.
 2. The process of claim 1 including disconnecting thefunctional output bond pad from the test input test pad during operationof the functional circuitry.
 3. The process of claim 1 includingdisconnecting the second positive power test pad during operation of thefunctional circuitry.
 4. The process of claim 1 including disconnectingthe second negative power test pad during operation of the functionalcircuitry.
 5. The process of claim 11 including forming the comparatorto be an analog comparator.
 6. The process of claim 1 including formingthe comparator to be a digital comparator.